German patent application DE 92 16 10236788 A1 describes a scale in the form of an amplitude grating. This amplitude grating includes a substrate having a reflective layer thereon. This reflective layer is followed by a transparent spacer layer which, in turn, has a partially transparent layer deposited thereon. This partially transparent layer is patterned by partial removal. Light incident on this partially transparent layer is partially reflected and partially transmitted. The reflected light is extinguished by destructive interference with the transmitted light that is reflected back from the lower reflective layer, so that the scale appears dark here, while in regions where the partially transparent layer is absent, the scale appears bright.
To this end, (under the simplifying assumption of a phase jump of exactly π at the interfaces air/partially transparent layer, air/spacer layer, as well as spacer layer/reflective layer, and negligible absorption), the transparent spacer layer must cause a phase jump of π; i.e., (given perpendicular incidence of light) must have a thickness which, multiplied by the refractive index of the spacer layer, is equal to one-fourth of the wavelength of the light used for scanning. However, in the bright regions, the inventors have recognized that a certain portion of light is also reflected at the interface between the air and the spacer layer, so that this reflected light also interferes destructively with the light that is reflected at the reflective layer. As a result, the bright regions appear less bright and the bright/dark contrast of the amplitude grating is reduced. In addition, the partially transparent layer is made of a very thin (a few nanometers) metal layer, which is exposed to both chemical and mechanical influences. Therefore, the inventors have recognized that, under certain circumstances, it may be damaged or its optical properties may change.